1. The Field of the Invention
The present invention relates to communication cables. More specifically, the present invention relates to laser drivers for powering lasers within closed path digital optical cables.
2. The Relevant Technology
Electro-Optical Communication Technology
Networks employing fiber optic technology are known as optical communications networks. To communicate over a network using fiber optic technology, fiber optic components such as fiber optic transceivers are used to send and receive optical data. Generally, a fiber optic transceiver can include one or more optical subassemblies (OSA) such as a transmitter optical subassembly (TOSA) for sending optical signals, and a receiver optical subassembly (ROSA) for receiving optical signals. In particular, a typical TOSA includes an optical transmitter, such as a laser, for sending an optical signal. Many different types of lasers are known to those skilled in the art. One type of laser referred to as a vertical cavity surface emitting laser (VCSEL) emits light in a single direction through an upper surface of the laser structure.
Conventionally, output characteristics of a TOSA must be controlled by complex circuitry and/or programming. Conventional laser drivers typically include control and setup circuitry to control and drive the laser over the intended application temperature range. In addition, a variety of industry performance and safety optical standards often apply that must be adhered to. As a result, conventional laser drivers generally include some form of closed path power control circuit, temperature programming of the laser bias current, etc.
Conventional TOSAs also may include a monitor, such as a photodiode, that generates feedback concerning performance characteristics of the laser. The monitoring function may be necessary in order to maintain required modulation rates and on/off extinction ratios. Average power techniques may be used to control the power output of the laser. For example, an automatic power-control (APC) feedback loop may be incorporated to maintain a constant average optical output power from the laser over temperature and lifetime. The laser driver circuitry is also often designed to compensate for signal degradation and parasitics using methods such as peaking on electrical signals or use of passive electrical matching networks. Control over laser output characteristics becomes of increased importance as rates of data transmission increase.
Optical networks must also meet various industry standards. For example, there are Fibre Channel and Ethernet industry standards. The International Electrotechnical Commission (IEC), for example, sets forth standards for hazardous light emissions from fiber optic transceivers. One standard, IEC 825, defines the maximum light output for various hazard levels. Thus, conventional laser drivers include circuitry and/or components, such as a monitor, in order to adhere to such industry standards.
Several safety precautions are incorporated into current transceiver designs to satisfy the various safety standards. For example, a common safety precaution is single-point fault tolerance, whereby one unplanned short, open, or resistive connection does not cause excess light output. Even with the various safety precautions included by manufacturers to prevent violations (typically constituting monitoring circuitry and control components), manufacturers and consumers must still be aware of various levels of fault tolerance required by the particular application and ensure that the components are compliant.
Controlling output parameters and meeting industry standards can be particularly difficult for optical transmission devices that use VCSELs as their optical source. Laser drivers for high-speed VCSEL applications typically contain a bias generator, a laser modulator, and comprehensive safety features. Circuitry and electronic components are typically used for automatic power control (APC), which adjusts the laser bias current to maintain average optical power over changes in temperature and laser properties. The laser driver can accommodate common cathode and differential configurations. Adjustable temperature compensation is typically provided to keep the optical extinction ratio of the VCSEL within specifications over the VCSEL's operating temperature range. The laser drivers for VCSELs can also include circuits for detecting safety faults that can cause hazardous light levels or violate other multisource agreements.
A modulator can further include circuitry for peaking compensation. Other circuitry can prevent current spikes to the laser during power-up or enable, further ensuring compliance with various safety standards. The modulation circuitry can include an input buffer, a current mirror, and a high-speed current switch. The modulation circuitry is typically also controlled based on the temperature of the laser and according to other external program codes to insure that the laser meets the various safety standards.
As a result of the foregoing, laser drivers for conventional optical laser signal transmission devices have required a significant number of passive components and/or programming for their use. These additional components add cost and complexity to the devices. For example, conventional laser drivers, such as the MAX3740A made by Maxim Integrated Products, require several thousands of transistors (e.g. 3806 transistors for the MAX3740A implementation).
Introduction to Digital Consumer Electronics
Digital consumer electronics, such as digital video displays, digital video disk (DVD) readers, flat screen computer monitors, high definition television (HDTV), digital plasma screens, digital audio readers, digital audio encoders, digital audio amplifiers, and digital audio processing devices have become of increased popularity. As the amount of data transferred between digital components expands to accommodate the desire for greater resolution, size, and quality, the need for high speed data transfer of digital data also increases. Several standards supporting data transfer to digital consumer electronic devices have been developed, but many have not adequately addressed the high bandwidth and high resolution needs of emerging products. For example, two current standards implemented for transmission of digital video and/or digital audio include the digital video interface (DVI) standard and high definition multimedia interface (HDMI) standard. Both the HDMI standard and the DVI standards are based on transmission minimized differential signaling (TMDS), Silicon Image's high-speed, serial link technology.
1. DVI Technology
DVI is a display interface developed by the Digital Display Working Group (DDWG). The DVI specification can provide a high-speed digital connection between DVI digital source devices (i.e. compliant DVI digital video processing devices) and DVI digital sink devices (i.e. compliant DVI digital video display devices). One common implementation of DVI is an interface for a computer having a video controller card and a digital display device (e.g. CRT, LCD, projector, etc.) having a display controller.
The DVI interface standard and description are contained within the publication entitled Digital Visual Interface, Revision 1.0, published by the Digital Display Working Group on Apr. 2, 1999, the contents of which is hereby expressly incorporated herein by reference. DVI utilizes a high-speed serial interface and TMDS to send data to the DVI sink device. TMDS conveys data by transitioning between “on” and “off” states. An encoding algorithm uses Boolean exclusive OR (XOR) or exclusive NOR (XNOR) operations applied to minimize the transitions to avoid excessive electromagnetic interference (EMI) levels in the DVI cable. An additional operation is performed to balance the DC signal.
The DVI connector has 24 pins that can accommodate up to two TMDS links. The basic TMDS transmission line is made up of three data channels and a clock channel. Data comprise 8-bit pixels in each of three channels (R/G/B). In some instances, a pair of conventional TMDS lines may be used to achieve higher data rates. In addition to the TMDS data channels and clock channels, the DVI includes a 5V DC power source, and a hot plug detect channel. The DVI-I combined digital and analog pin assignments are similar to the DVI-D digital only interface pin assignments, but further includes several pins for transmission of an analog signal.
FIG. 1 illustrates the typical flow of data from a graphics controller 120 of a DVI source device 125, such as a digital video processing device, through the TMDS links 130 and to the display controller 135 of a DVI sink device 140, such as a digital video display device. In this process, incoming 8-bit data are encoded into 10-bit transition-minimized, DC-balanced characters. The first eight bits are encoded data, and the ninth bit identifies whether the data was encoded with XOR or XNOR logic; the tenth bit is used for DC balancing.
Due to the defined properties of the DVI interface, DVI cables having copper electrical cables may be limited to a length of about 3-5 meters. This limited length reduces the number of potential applications that can utilize DVI cables. For example, the length limits remote placement of digital video components.
Typical DVI cables having copper electrical links are also limited in bandwidth and data transfer rates. DVI data rates typically range from 22.5 mega pixels per second (Mpps) to 165 Mpps (up to 1.65 Giga bits per second). Because TMDS conveys data by transitioning between “on” and “off” states, EMI levels in the DVI cable can also limit the speed at which data may be transferred.
Further, although DVI is a standard interface, some digital video processors and digital video displays may be incompatible or incapable of interoperation with one another. Thus, at least in some environments, bidirectional communication for reconfiguring a digital video processor and/or digital video display would be desirable. Unfortunately, configuration data are typically not transmitted. Further, many DVI interfaces lack sufficient connectivity to transmit data (e.g. configuration data) from the digital video display to the digital video processor. As a result, a digital video processor and a digital video display may remain incompatible.
2. HDMI Technology
HDMI is backward compatible with devices incorporating the DVI standard. HDMI is based on the TMDS serial link technology. HDMI technology supports standard, enhanced, or high-definition video, plus multi-channel digital audio on a single cable. It transmits Advanced Television Systems Committee's (ATSC's) HDTV standards and supports 8-channel digital audio with 5 Giga bits per second of bandwidth. The HDMI technology, functionality, and hardware is disclosed in the “High-Definition Multimedia Interface” specification Version 1.1, May 20, 2004, by HDMI Licensing, LLC, the contents of which are hereby expressly incorporated by reference herein in their entirety.
The HDMI interface is provided for transmitting digital television audiovisual signals from DVD players, set-top boxes, and other audiovisual consumer electronic source devices to HDMI consumer electronic sink devices, such as television sets, projectors, and other audio visual devices. HDMI can carry multi-channel audio data and can carry standard and high definition consumer electronics video formats. Content protection technology is also available. HDMI can also carry control and status information in both directions.
Referring to FIG. 2, an HDMI block diagram is shown where a standard HDMI cable includes four differential pairs 201-204 that make up the TMDS data and clock channels, referred to collectively as HDMI TMDS links 200. These links 200 are used to carry video, audio and auxiliary data. In addition, HDMI carries a VESA Display Data Channel (DDC) 205. The DDC 205 is used for configuration and status exchange between an HDMI source 210 and an HDMI sink 215. The optional CEC protocol line 220 provides high-level control functions between all of the various audiovisual products in a user's environment.
Audio, video and auxiliary data are transmitted across the three TMDS data channels 201-203. Video pixel clock data are transmitted on the TMDS clock channel 204 and are used by an HDMI receiver 230 as a frequency reference for data recovery on the three TMDS data channels 201-203. Video data are carried as a series of 24-bit pixels on the three TMDS data channels 201-203. TMDS encoding converts the 8 bits per channel into a 10-bit DC-balanced, transition minimized sequence, which is then transmitted serially across the HDMI TMDS data channels 201-203 at a rate of 10 bits per pixel clock period. Video pixel rates can range from 25 MHz to 165 MHz. The video pixels can be encoded in either RGB, YCBCR 4:4:4 or YCBCR 4:2:2 formats.
In order to transmit audio and auxiliary data across the links 200, HDMI uses a packet structure. In order to attain higher reliability of audio and control data, these data are protected with an error correction code and are encoded using a special error reduction coding to produce the 10-bit word that is transmitted. Optionally, HDMI can carry one such stream at sample rates up to 192 kHz or from two to four such streams (3 to 8 audio channels) at sample rates up to 96 kHz. HDMI can also carry compressed (e.g. surround-sound) streams. The DDC channel 205 is used by the HDMI source device 210 to read the HDMI sink device's 215 Enhanced Extended Display Identification Data (E-EDID) to discover the sink device's 215 configuration and/or capabilities. The HDMI source device 210 reads the sink device's 215 E-EDID and delivers only the audio and video formats that are supported by the sink device 215. In addition, the HDMI sink device 215 can detect Info Frames and process the received audio and video data appropriately.
A digital consumer device's external HDMI connection is embodied by two specified HDMI connectors, Type A or Type B. These connectors can be attached directly to the device or can be attached via a cable adapter that is shipped with the device. The Type A connector carries all required HDMI signals, including a single TMDS link. The Type B connector is slightly larger and carries a second TMDS link, which is necessary to support very high-resolution computer displays requiring dual link bandwidth.
The CEC protocol line 220 is optionally used for higher-level user functions such as automatic setup tasks or tasks typically associated with infrared remote control usage. The Type A connector carries only a single TMDS link and is therefore only permitted to carry signals up to 165 Mpps. To support signals greater than 165 Mpps, the dual-link capability of the Type B connector is used.
The input stream to the HDMI source's transmitter 235 from the HDMI source's controller 240 will contain video pixel, packet, and control data. The packet data can include audio data, auxiliary data, and associated error correction codes. These data items are processed in a variety of ways and are presented to the HDMI source's transmitter 235 as either 2 bits of control data, 4 bits of packet data or 8 bits of video data per TMDS channel. The HDMI source controller 240 encodes one of these data types or encodes a Guard Band character on any given clock cycle. The stream of TMDS characters produced by the transmitter 235 is serialized for transmission on the TMDS data channels 201-203.
These current cables and solutions, as well as others, are limited in many ways in their capabilities to carry digital video and/or audio signals. For example, these digital video and/or audio cables are limited in bandwidth and distance in which they can carry TMDS signals. One solution to the problem of limited length of these cables is a repeater, which is a device with a retransmission function for extension or distribution of digital video and/or audio signals from cables such as DVI and HDMI cables. The circuitry of a repeater can retrieve, equalize, amplify, and re-transmit the digital video and/or digital audio signals into another length of cable. A repeater may be capable of transmitting digital video and/or audio signals to about 25 or 35 meters in some instances. However, a repeater can be quite expensive, add additional hardware and circuitry, require additional cables for the extension, and even still be relatively limited in distances to which the repeater can transmit digital video and/or audio signals and bandwidth of the cables. Therefore, repeaters have not provided a desired solution to many of the problems currently experienced with these cables, but rather have tried to mitigate the limitations of such cables.
Thus, for these reasons, as well as others, what would be advantageous are simplified and cost effective laser drivers for optical cables.